High-Strength Carbon Steel Bolt Failures due to Hydrogen Embrittlement (HE)
A bolt, or threaded mechanical fastener, is used to transmit its induced tension to components to be connected so that they can react reliably without failure under the imposed loads. The amount of induced tension depends upon the thread helix angle, thread diameter, tightening torque and friction coefficient at the sliding interfaces during tightening.
Thus the designer of a bolted connection has to optimise the number and size of bolts to achieve this objective. For cost, weight or space reasons, the tension may be such that bolts with a high minimum strength are demanded to avoid failure at the section of maximum stress, normally the thread root though some special bolts are waisted. This may also be a response to in service failures where constraints of number and size direct a bolt of higher strength.
Bolts are designated by grades that relate to the tensile strength properties of the bolt, i.e. the amount of applied load a fastener can be subjected to before failure occurs. The proof load is also an important parameter for consideration; this is the load a bolt can withstand without permanent deformation, i.e. elastic stretch. Applied loads that go beyond the proof load will result in permanent deformation or elongation to the bolt. Further increases in applied load will then take the bolt to the point at which failure occurs; this is known as the ultimate tensile strength (UTS).
The grade of a bolt is generally marked on the head. Older metric systems utilise a series of radial lines to indicate grade whilst imperial bolts are identified by a numeric system, see Table 1. Specified values for ach bolt grade are the minimum expected properties. Another load commonly acting on a bolt is shear stress, bolt designations however do not provide any information on resistance to shear and particular applications should factor in the possibility of shear along with other loading such as fatigue and torsion.
However, there are downsides to the selection of high strength bolts which are discussed further here. Careful selection of bolt grade must be considered to avoid catastrophic failure. For example, in an application where the load results in a maximum stress of 900 MPa, a Grade 10.9 bolt may suffice to avoid failure in service, however the conservative approach would be to provide a generous factor of safety and select a Grade 12.9; this can often prove catastrophic.
An increase in bolt grade will provide margins for yield strength and UTS, whilst the fatigue strength would also increase, providing the environmental effects such as corrosion do not provide crack initiators for materials that are becoming increasingly notch sensitive. Furthermore, the increased hardness of the material introduces other possible problems during both manufacture and in service, namely hydrogen embrittlement or HE.
Hydrogen Embrittlement (HE)
A common, but still not completely understood, failure mechanism in high strength fasteners is HE. This mechanism sounds like an informative description; embrittlement of the material due to hydrogen but the term is so often greeted with worry and despair.
So, why is the mechanism such as phenomena?
The problem arises due to the migration of hydrogen into the structure being possible at the manufacturing stage or through corrosion in service, or a combination of both. Identification of the presence of hydrogen within the structure is difficult, and made more so due to the fact that following failure, the hydrogen can exit the structure as easily as it entered.
So, how can we determine if HE is a problem?
If the fastener in question is manufactured from carbon steel and has a hardness values above 320 on the Vickers scale (HV) and if the fastener uses a protective coating for corrosion resistance, then, HE should be considered as a possible failure mechanism.
Materials with a hardness below the 320 HV threshold are not considered a risk for HE. If the bolt does not undergo electroplating, or, is manufactured from stainless steel, then failure due to HE is also very unlikely.
Identification
The stigma of fastener failure through HE is not unjustified; unlike mechanisms such as corrosion or fatigue, HE is barely detectable from a visual perspective and occurs suddenly. The only positive aspect is that it is often detected at installation when the torque is applied to the bolt, or, can even occur before fitment whilst in storage where embrittlement was a result of the manufacturing process.
Fracture surfaces tend to exhibit a granular morphology with no evidence of ductility such as narrowing or necking of the bolt or coarse jagged surfaces such as those resulting from pure overload. Fractures may also possess some surface corrosion which may not have resulted in a large amount of material loss, this indicates that the source of hydrogen was evolved during the corrosion process.
Figure 2: (a) Ductile overload failure mechanism and (b) intergranular hydrogen embrittlement.
A granular fracture surface is the result of the hydrogen migrating along the weaker grain boundaries of the material resulting in an intergranular propagation mechanism. Hydrogen, the first element on the periodic table, has the smallest atom size of all elements can easily pass through the steel. The weakened grain boundaries cause a decrease in the ductility of the material, leading to failure of the material under loading well below that expected. It is the decrease in yield strength that often highlights the mechanism upon installation.
Figure 3: Scanning Electron Microscopy image of intergranular hydrogen embrittlement mechanism.
Manufacturing
A number of steps in the manufacturing stage can lead to hydrogen diffusion into the steel, with consideration of the possibility; a combative process can be adopted at each stage however to avoid the dreaded HE.
Some of the processes that pose risks include, cleaning and pickling, heat treatment such as carburising or during electroplating. Other causes of embrittlement during manufacture include hydrogen diffusion during thread rolling or machining.
In most cases a post fabrication heat treatment (baking) process can utilised to allow the trapped hydrogen to escape the structure. The process is a temperature-time dependant one and should apply to specific materials and mechanical properties.
Temper embrittlement
It should be noted however that heat treatments themselves may introduce hydrogen into the structure; this is known as temper embrittlement (TE). TE occurs in low-alloy steels when the cooling rate within the range of 250-400°C is too slow, which increases the ductile-brittle transition temperature of the material.
Therefore, any fastener failures that exhibit intergranular cracking should include as assessment of the manufacturing process to locate the source of the hydrogen, particularly in Cr-Ni and Cr-Mn steels. Often temper embrittlement has no effect on the hardness or tensile strength of the bolt but can be identified by a reduction in the impact toughness properties. As with other forms of embrittlement however, a heat treatment process can often reverse TE. Furthermore, certain steel composition will avoid the risk of embrittlement, i.e. <0.5% Mn content.
Corrosion
We have considered the cleaning and pickling process, made attempts to avoid hydrogen entrapment beneath the electroplating, and carried out a post fabrication heat treatment to remove any diffused hydrogen. We are now completely protected against HE failures aren’t we? Afraid not, the possibility of hydrogen evolution through corrosion mechanism still needs to be addressed.
HE due to corrosion is often easier to identify. Typically, the bolt will show some form of corrosion attack, general corrosion or pitting, which does not have to be a considerable amount, and it is more than likely that the bolt would have been in service of a period of time before failure.
Corrosion induced HE is somewhat of a minefield where terminologies exist. A common form of fastener failure is stress corrosion cracking (SCC). A well-researched and understood form of corrosion, this confusingly however falls under the `umbrella` of HE. SCC is difficult to detect visually and occurs unexpectedly normally below the expected yield of a material. The crack propagation mechanism differs from pure HE however. Branched fir tree cracks are typical of SCC. Propagation is generally intergranular, but in some situations transgranular cracking can be observed. Often however, chlorine is considered the driving force of cracking, which will only occur to a susceptible material that suffers corrosive attack under stress. Comparison of the two fractures can easily determine which mechanism has resulted in fracture.
Figure 6: SCC of bolt exposed to sea water spray – branched cracking and corrosion.
So, why is SCC a form of hydrogen embrittlement?
Some of the problems relating to the term HE, is the vastness of mechanism it covers, or more accurately the simplicity of the term. Hydrogen is an abundant element throughout the universe and it can be a very difficult task to avoid. The cathodic process during corrosion unavoidably evolves atomic hydrogen. The hydrogen evolution in SCC occurs at the corroding crack tip.
Engineers can now rest easy, to prevent HE in service due to corrosion, the engineer can simply add a cathodic protection (CP) system and get a good night’s sleep… Wrong!
Incorrectly applied CP systems can actually generate hydrogen and cause material embrittlement and only experts in the field should design a CP system specifically for the project in question.
Material Selection
All of the terminologies and factors to consider in order avoiding HE can be inconsequential if the correct material is chosen initially. Many cases have shown that often safety factors are too large and hard, high strength material is chosen to provide cover for possible overload events. By selecting a material that has a reduced hardness <320 HV, the susceptibility of HE is eradicated.
A fitness-for-purpose case study details the failure of a large number of high tensile strength bolts through HE which have been avoided by simply employing the use of a lower grade bolt.
For further information regarding embrittlement processes and bolt related failures, or, to disucss any other failure investigations please contact Steve Gill on 01925 843428 or This email address is being protected from spambots. You need JavaScript enabled to view it..